The present invention relates to a display control circuit and, more particularly, to a display control circuit for changing plural types of colors or densities at high speed.
In a conventional display device, a rewritable color value register is used in a color designation circuit and the color designation circuit is arranged as an IC (Integrated Circuit), as is known to those skilled in the art.
An example of a conventional display device shown in FIG. 1 will be described below.
Referring to FIG. 1, a video memory 3 stores display data assigned with a plurality of bits corresponding to each pixel of a display screen of the display device. Display data supplied from a processor 1 is written in the video memory 3 under the control of a write controller 2. The contents of the video memory 3 are read out under the control of a read controller 4 and are output to a color designation circuit 12. The color designation circuit 12 is an IC comprising a rewritable color value register 5 and a selector 6 for selecting outputs from the color value register 5 in accordance with color signal information of the display data read out from the video memory 3 by the processor 1. G, R and B color signals from the selector 6 are output to G, R, and B drivers 10a, 10b, and 10c for a CRT 11, respectively.
The storage contents of the color value register in the color designation circuit 12 are updated in accordance with the display data from the processor 1, and therefore the display color on the display screen of the CRT 11 can be immediately changed without updating the contents of the video memory 3.
FIG. 2 is a block diagram showing an example of a display device including a plurality of color designation circuits so as to increase the number of colors to be simultaneously displayed.
The same reference numerals as in FIG. 1 denote the same parts in FIG. 2, and a detailed description thereof will be omitted. A video memory 3a has the same function as that of the video memory 3, and an extended video memory 3b stores element number signals of display data. Each of the video memories 3a and 3b constitutes a memory for storing display data assigned with a plurality of bits corresponding to each pixel of a display screen of the display device. Each of color designation circuits (ICs) 12a to 12d has the same function as that of the color designation circuit 12. Outputs from the color designation circuits 12a to 12d are selected by an element number signal read out from the extended video memory 3b, and the selected outputs are supplied to color drivers 10a, 10b, and 10c.
In the conventional display device described above, in order to increase the number of colors to be simultaneously displayed, the number of color value registers in the color designation circuits must be increased. However, since the color designation circuits are ICs, the number of registers in the color designation ICs is fixed. Therefore, the number of color designation ICs must be increased.
When a plurality of color designation ICs are used, a selector 13 (FIG. 2) is required to select the outputs from the color designation circuits (ICs) 12a to 12d. When the number of color designation ICs is increased, the circuit arrangement of the selector 13 is inevitably complicated. More particularly, in an arrangement wherein n color designation ICs each capable of simultaneously displaying any m of 2 l colors are used to simultaneously display m .times.n colors of the 2 l colors, l selectors 13 each arranged to select one of the n inputs must be arranged.
The selectors 13 cannot be distributed in the color designation ICs due to contention of outputs from the ICs. When the selectors 13 are arranged as one IC, the number of color designation ICs is determined by the number of inputs of the selector IC, and the extension of the circuit cannot be easily performed.